Problem: $ { {-1} \times \left[\begin{array}{r}{1} \\ {2} \\ {1}\end{array}\right] = {?} } $
Answer: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{-1}\times{1} \\ {-1}\times{2} \\ {-1}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{-1} \\ {-2} \\ {-1}\end{array}\right]}$